Zwitterionic per- and polyfluoroalkyl substances (PFAS) are synthetic chemicals that contain both positive and negative charges, resulting in net-neutral molecules. These compounds are incorporated into many consumer and industrial products and considered precursors of anionic perfluoroalkyl acids, which are regulated in drinking water. Despite their prevalence, zwitterionic PFAS are less frequently monitored than perfluoroalkyl acids in environmental waters; furthermore, no passive sampling devices are specifically designed for the measurement of zwitterionic PFAS. To address this knowledge gap, we evaluated the uptake of zwitterionic PFAS by anion- and cation-exchange membranes, as the active layer of novel passive samplers, under variable water quality conditions. In the experiments, 1×1 cm2 membrane coupons were placed in 100 mL of water containing three zwitterionic PFAS, namely 6:2 FTAB, 8:3 PFAQA, and N-TAmP-FHxSA, and two anionic PFAS, PFOA and PFOS. The time to equilibrium was determined in PFAS-spiked water (10 mM NaCl, pH 7) by sacrificially sampling at regular intervals (0, 1, 2, 3, 6, 12, 24, 48, 72 h) and measuring the membrane-phase PFAS concentrations. Those data were used to calculate first-order rate constants for PFAS uptake from the bulk solution and determine the time to equilibrium (24-72 h); as a result, batch equilibrium tests were conducted for at least 168 h. While 6:2 FTAB, 8:3 PFAQA, and N-TAmP-FHxSA are primarily zwitterions at pH 4-9, these compounds exist as cations at pH < 3; furthermore, 6:2 FTAB is an anion at pH > 10. We hypothesized that these speciation profiles and other water quality parameters would influence the equilibrium uptake of PFAS in the two ion-exchange membranes. To test this hypothesis, we varied solution pH (2, 5, 12), PFAS concentration (100, 250, 500, 1000 μg/L), and salinity (10, 100, 600 mM) and measured the aqueous- and membrane-phase PFAS concentrations after one week of mixing. The results confirmed that the PFAS end group played a key role in determining the preferred membrane. While 6:2 FTAB, which has a negative end group, was preferentially accumulated in the anion-exchange membrane, 8:3 PFAQA and N-TAmP-FHxSA, which have positive end groups, favored the cation-exchange membrane. PFOA and PFOS were only detected in the anion-exchange membrane, as expected. The results from variable pH tests agreed with our speciation-based hypothesis, with 6:2 FTAB, 8:3 PFAQA, and N-TAmP-FHxSA showing a greater preference for the cation-exchange membrane at pH 2; furthermore, the anion-exchange membrane exhibited better 6:2 FTAB uptake at pH 12. Salting-out phenomena led to greater PFAS accumulation in membranes submerged in solutions with increased salinity, in agreement with our previous findings for anionic PFAS. Overall, these results improve our understanding of zwitterionic PFAS interactions with ion-exchange membranes, enable the development of field-ready passive samplers for both zwitterionic and anionic PFAS, and provide key insights into sorption processes for selective treatment of zwitterionic PFAS.
